Increased hydrogen peroxide (HOOH) is thought to occur under conditions of tissue reperfusion with oxygenated media after hypoxia. We were first to show that when modified hemoglobins are challenged with HOOH, damaging and potentially toxic species are formed including the ferryl protein radical (Fe+4). We have recently published a mechanism, based on singular least squares kinetic analysis of time-based spectral data, to describe fully the interactions of a number of modified hemoglobins with HOOH. We have suggested that besides modifying ligand interactions, site specific modification can also affect the tendency of hemoglobin to undergo oxidative modification and the production of the ferryl species. Cycling between the ferric and the ferryl heme in the presence of HOOH can result in apparent peroxidase activity, and that the level of activity also being dependant on structural modification of the protein. In another set of experiments the ferric forms of hemoglobin AO and its cross-linked derivatives were added to suspensions of liposomes. Both the hemoprotein and the lipid constituents of the system underwent chemical change. Differences in the pro-oxidant activities among these hemoglobins were found to be either to the differential population of their ferryl intermediates or disparate dimerization and transfer of heme into membrane with subsequent heme degradation. Since the vascular endothelium is the first in line to exposure of circulating hemoglobin-based blood substitutes, we investigated the effects of a variety of modified hemoglobin on the integrity of bovine aorta endothelial cells (BAEC) and on the HOOH mediated cytotoxicity. We found that unmodified hemoglobin AO to be protective against peroxide-induced endothelial cell necrosis, probably due to its peroxidase activity. Intra- and intermolecular cross-linked hemoglobin, however, did not protect against peroxide-induced endothelial cell damage.